I personally believe that the little lady is a she, based on the fact my baby girl is about a year old and has a bump around the same size. Boys have very noticeable cloacae, so I would feel quite safe in saying she is female at the moment. But please, be warned - I have heard of axolotls suddenly changing sex overnight (because they are actually male, and 'late bloomers') at a year old.
If the bump is still that size in two months, I would confidently say she's a ma'am.